Cold protective clothing, for example, overalls and jackets, are designed to protect those working or playing in low temperature environments against substantial body heat loss. Referring now to FIG. 1, it can be seen that such garments are constructed from a fabric system 10 which typically includes an outer shell 11, an insulative layer 12 of batting, down, other insulation, and a lining layer 14. With this configuration, the heat loss from the body through the garment layers is slowed by the air spaces of insulative layer 12. Because the thermal insulation of the garment is provided mainly by the batting and is directly related to the volume of air trapped therein, insulative capability of such garments is typically increased by increasing the thickness of the insulative layer.
Clothing utilizing such conventional construction for cold weather applications have some deficiencies, however. For example, increasing the thickness of the insulative layer can make cold weather clothing so bulky as to be impractical to wear when tasks have to be performed by the wearer. Also, conventional cold weather clothing can become uncomfortable when the wearer is involved in alternating periods of inactivity and intense activity. For example, when a person skiing down a ski slope is wearing a conventional winter jacket skis, the person's metabolic heat rate increases substantially. This heat cannot be released as required to maintain comfort, because the insulation layer works against such release. The person tends to overheat and may perspire. The perspiration can wet the liner and the insulative layers. Then, when the person stops skiing, such as when a skier sits in a chair lift and rides to the top of the ski slope, insulative capabilities of the jacket are decreased by the dampness and the skier becomes chilled during the chair lift ride. Fundamentally, the conventional cold weather clothing discussed above has a generally static response, and is unable to response variously to changing wearing conditions.
New materials have been developed in an attempt to address special clothing and other thermal regulating system requirements. For example, microencapsulated phase change materials have been described as a suitable component for substrate coatings when exceptional heat transfer and storage capabilities are desired. In particular, U.S. Pat. No. 5,290,904 for "Fabric with Reversible Enhanced Thermal Properties" to Colvin, et al., incorporated herein by reference teaches that substrates coated with a binder containing microcapsules filled with energy absorbing phase change material enables the substrate to exhibit extended or enhanced heat retention or storage properties. Substrates coated with a binder containing microencapsulated phase change materials are referred to herein as microPCM-coated substrates.
Also by way of example, microencapsulated phase change materials have been described as a suitable component for inclusion in fibers, when exceptional heat transfer and storage capabilities are desired. In particular, U.S. Pat. No. 4,756,958 for "Fiber with Reversible Enhanced Thermal Properties and Fabrics Made Therefrom" to Bryant, et al., also incorporated herein by reference, teaches that a fiber with integral microspheres filled with phase change material or plastic crystals has enhanced thermal properties at predetermined temperatures. This patent further teaches that such fibers may be woven to form a fabric having the enhanced thermal storage properties, and that articles of manufacture may be formed therefrom. Fabrics manufactured from such fibers are referred to herein as microPCM-containing fabrics.
Generally speaking, phase change materials have the capability of absorbing or releasing thermal energy to reduce or eliminate heat transfer at the temperature stabilizing range of the particular temperature stabilizing material. The phase change material inhibits or stop the flow of thermal energy through the coating during the time the phase change material is absorbing or releasing heat, typically during the material's change of phase. This action is transient, i.e., it will be effective as a barrier to thermal energy until the total latent heat of the temperature stabilizing material is absorbed or released during the heating or cooling process. Thermal energy may be stored or removed from the phase change material, and can effectively be recharged by a source of heat or cold. By selecting an appropriate phase change material, a substrate can be coated or a fiber manufactured incorporating a phase change material, for use in a particular application where the stabilization of temperatures is desired.
Exemplary paraffinic hydrocarbon phase change materials suitable for use in the coatings or in fibers are shown in Table I, with the number of carbon atoms in such materials directly related to the respective melting and crystallization points.
TABLE I No. Crystallization Melting Compound Carbon Atoms Point Point n-Eicosane 20 30.6.degree. C. 36.1.degree. C. n-Octadecane 18 25.4.degree. C. 28.2.degree. C. n-Heptadecane 17 21.5.degree. C. 22.5.degree. C. n-Hexadecane 16 16.2.degree. C. 18.5.degree. C.
The patents identified above teach how phase change materials such as the above-listed paraffinic hydrocarbons are preferably formed into microspheres and encapsulated in a single or multi-layer shell of gelatin or other material. Encapsulated microsphere diameters of 1 to 100 microns are preferred, most preferably from 10 to 60 microns. Microspheres may also be bound in a silica matrix of sub-micron diameters.
Newer fabrics incorporating phase change materials as identified above are beginning to be individually incorporated into commercially available clothing. However, a configuration which is especially adapted to provide a superior thermal response in low temperature conditions where variable activity levels or weather conditions occur is not yet available. Thus, there remains a continuing need for materials which can provide a dynamic thermal response.
It is against this background that the significant improvements and advancement of the present invention have taken place in the field of substrates incorporating energy absorbing and releasing temperature stabilizing phase change materials.